1. Field of the Invention
This invention relates to methods of making micromechanical structures having at least one lateral, small gap therebetween and micromechanical devices produced thereby.
2. Background Art
Vibrating mechanical tank components, such as crystal and SAW resonators, are widely used for frequency selection in communication sub-systems because of their high quality factor (Q's in tens of thousands) and exceptional stability against thermal variations and aging. In particular, the majority of heterodyning communication transceivers rely heavily on the high Q of SAW and bulk acoustic mechanical resonators to achieve adequate frequency selection in RF and IF filtering stages and to realize the required low phase noise and stability in their local oscillators. In addition, discrete inductors and variable capacitors are used to properly tune and couple the front end sense and power amplifiers, and to implement widely tunable voltage-controlled oscillators.
At present, the aforementioned resonators and discrete elements are off-chip components, and must interface with integrated electronics at board level, often consuming a sizable portion of the total sub-system area. In this respect, these devices pose an important bottleneck against the ultimate miniaturization and portability of wireless transceivers. For this reason, many research efforts have been focused on strategies for either miniaturizing these components or eliminating the need for them altogether.
The rapid growth of IC-compatible micromachining technologies that yield micro-scale, high-Q tank components may now bring the first of the above strategies closer to reality. Specifically, the high-Q RF and IF filters, oscillators, and couplers, currently implemented via off-chip resonators and discrete passives may now potentially be realized on the micro-scale using micromachined equivalents based on a variety of novel devices, including high-Q, on-chip, vibrating mechanical resonators, voltage-tunable, on-chip capacitors, isolated, low-loss inductors, microwave/mm-wave medium-Q filters, structures for high frequency isolation packaging, and low-loss mechanical switches. Once these miniaturized filters and oscillators become available, the fundamental bases on which communication systems are developed may also evolve, giving rise to new system architectures with possible power and bandwidth efficiency advantages.
Prototype high-Q oscillators featuring lateral comb-driven micromechanical resonators integrated together with sustaining electronics, all in a single chip, using a planar process that combines surface-micromachining and integrated circuits, have been demonstrated. The gap between the electrodes and the structure of the comb-driven micromechanical resonator is limited by lithography capability. Therefore, a submicron gap is very difficult to do. As the frequency of the resonator goes higher, the size of the resonator becomes smaller. So the electromechanical coupling is smaller. In order to increase the electromechanical coupling, a small-gap between the electrode and the structure is necessary. Although the capacitive gap of vertical micromechanical resonators, which is defined by the thickness of a sacrificial layer, can be very small, clamped-clamped beam vertical micromechanical resonators suffer from lower Q due to anchor dissipation. Also, it normally has only one port which limits its application range. Lateral resonators, on the other hand, have advantages of greater geometric design flexibility and more ports than normally attainable via vertical resonators. However, the electrode-to-resonator gap for capacitively-driven lateral resonators has historically been implemented via lithography and etching, and this greatly limits the degree by which the electrode-to-resonator gap spacing can be reduced.
In order to increase the electromechanical coupling for a lateral micromechanical resonator, a process to form a lateral submicron gap between an electrode and the resonator structure, without the need for advanced lithography tools, is desired.